Achievement of quality control in small incandescent lamps is a goal of long standing. The small size of the envelope, the leads the filament, makes the assembly subject to forces on sealing that are out of proportion to the structural integrity resulting from the small size of the components.
In one well-known technique for constructing flashlight bulbs, a pressed and sintered glass bead was used in U.S. Pat. No. 4,618,799 which is incorporated herein by reference. The glass beads were tablet shaped with an outer periphery which was in close proximity to the inside of the glass envelope. Upon sealing, and especially in the usual case where one side of the envelope and bead stack achieved a higher temperature than the other side, sealing would begin vertically along the stack, and spread to the opposite side.
In inserting the stack, a small clearance was used between the stack and the envelope which was small enough to insure easy and repeatable insertion, but large enough that “wetted” or liquid-liquid contact would of necessity start at one point and proceed around the periphery of the stack to the opposite side. This one point “wetting” and circumferential progression occurs naturally, and the use of a stack of pill shaped beads provided some help in stabilizing against the distortive “pull” of the heated softened envelope in the direction of the initial wetted contact between the beads and the envelope.
In normal assembly, enough of a clearance has to be provided between the bead stack and the glass envelope so that the glass envelope will have easy clearance upon placement over the bead, two wire and filament stack. Further, there must be enough clearance such that no interference will occur on heating. In normal assembly, the assembled bulbs are located in a fixture and heated as a group.
The only way to avoid heating one side of the envelope more rapidly than the other side would be to provide a completely centered assembly which is heated circumferentially radially evenly. However, heating each bulb in this way would drive up the cost significantly. Envelope sealing is typically done in batches of lamps arranged into a two dimensional array. Heating is accomplished as rapidly as is practicable and of necessity not concentrically centered on each bulb. Even if such concentric heating were attempted, sealing will typically begin at the point about the periphery of the concentrically inward element with the outer glass envelope in closest proximity to the outer glass envelope.
Contact is followed by “wetted pulling” of the envelope about the concentrically inward sealing elements. The use of a stack of beads as in the U.S. Pat. No. 4,618,799 patent acted to prevent wetted, cohesive, one-sided pulling of the envelope by relying upon the size and volume occupied by the inner stack of beads in the hope that more often than not, there would be enough structural integrity in the beads to resist any lateral pulling of the envelope.
The lateral pulling of the envelope using a stack of pill shaped beads is most pronounced when the spread of the wetted contact proceeds vertically upward along one side of the stack of beads and combines to pull the envelope laterally to one side. This effect is most pronounced where the wetted contact proceeds to the vertically uppermost bead, as it can exert the most direct and uncompensated force on the upper portion of the glass envelope.
Although this effect can be ameliorated in instances where three or four opposing radial zones occur simultaneously, the probability of a single sided vertical spreading zone occurring is statistically significant enough to result in waste significant rejection of lamps during inspection. Any inexpensive mechanism which would result in reduction of the occurrence of this phenomenon would increase the overall quality of lamps produced and reduce scrap.